From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 6B0D31A683E; Mon, 1 Jun 2026 14:32:30 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1780324351; cv=none; b=H0X0zw0RxWD/kIQQUN/o8ekT2J646D7L+UEfeFr1+JatvbcX5b+VRx9O5u1nnIrcJpdsW8wkXzJfrf0MpG99KCysq9cbxHfOkJkV7HlTNHEUS7I424wjKCOQPc0nLZwIfQ7ANJBYXNbrv+QGiRUYA2nA9NaFkm2zDt/uVfwABLA= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1780324351; c=relaxed/simple; bh=GJzfFVWDI/CqRtKwaoLte4IA2SBBd2lOqe7Tgz/TyRk=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=BAfofJ4Ll/uP3aDRUh9MkbzJTW94TbA/un9lFQGbjWABbn6kvg1XRu4xhpNSMbQhUqKS7vRptp8IgRpbgfD6D4P/CC7RVjZEscW2qSCd1+HNccHSz5TeOz0biSbP/bZFcBu3yZsfF6rsE4wn6tvmwNEb4iA7S1Qiq2BqVz3ACEY= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=lduKX4WA; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="lduKX4WA" Received: by smtp.kernel.org (Postfix) with ESMTPSA id BF67D1F00893; Mon, 1 Jun 2026 14:32:26 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1780324350; bh=wnW+JwsPZoEW9jymcriAzbpY9+T3DxT+086bhiowDn8=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=lduKX4WAFNqKtYvsGlo8Hyd5PJtGmk5207kJFlmngI1CuhMk8jRs8HLFI0NKytYAY 57IXIs7qFEcl91nOAAeZdXSFBMbmpLP8rOwB5+k7r/pwSSa9/6GMy/PAvg7gxCm8vW olFHNy0OUOHmcNzXEj3INi9xi+o2i197MnQnnuR1DDkEn53yzGKptZJLmgJDNkyt4Z cE30+1YtADBLYgBFikTWYODFwQMykd+i9KGjDWdgwei34amsraFp1CgnY9bcLSSvuR C/41bNxoN231Di3cc8//1sV62OgZ7Pvy+FxSAYCall8Sjrf/AiOYrPenoi8rJVsi2n zLA+YBuQEQofQ== Message-ID: Date: Mon, 1 Jun 2026 16:32:25 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm: don't allow empty relative nodemask in mpol_relative_nodemask() To: Yury Norov , Joshua Hahn Cc: Andrew Morton , Zi Yan , Matthew Brost , Rakie Kim , Byungchul Park , Gregory Price , Ying Huang , Alistair Popple , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Farhad Alemi , Waiman Long , Rasmus Villemoes , cgroups@vger.kernel.org References: <20260528124133.c88c27b11a8ea0ef05e494f7@linux-foundation.org> <20260529152616.2308736-1-joshua.hahnjy@gmail.com> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit >> >> Thank you for taking a shot at fixing the bug report, please let me know what >> you think! Have a great day : -) > > Hi Joshua. > > Indeed, quick and dirty shot. > > The problem is that nodes_fold() can't work with the sz == 0. In > other words, folding to a 0-bit bitmap is an error. We don't check > that on bitmaps level because it's an internal helper, and it's a > caller's responsibility to validate the parameters. > > nodes_onto(), or more specifically bitmap_onto(), is a different > story. In case of empty relmap, the function actually clears all the > bits in dst and returns. It's very weird that mpol_new_nodemask() (->create() callback) disallows empty nodemasks, but mpol_rebind_nodemask() (->rebind() callback) would allow empty nodemasks. I guess mpol_set_nodemask() could trigger it after doing the nodes_and(nsc->mask1, cpuset_current_mems_allowed, node_states[N_MEMORY]); And ending with an empty &nsc->mask1. The later "mpol_ops[pol->mode].create(pol, &nsc->mask2);" would reject it, but the division by zero could still happen. > > I see 2 options to move this forward. > > 1. Simply disallow empty relmap in mpol_relative_nodemask(). There's > no valid cases for it, AFAIK, so the nodes_fold() limitation looks > reasonable. We can consider it as a new policy. > > We've got 2 users for mpol_relative_nodemask(). In mpol_set_nodemask() > we can simply propagate the error; and in mpol_rebind_nodemask() we > can throw a warning and do nothing. Throwing a warning is really bad. We'd still end up with an empty nodemask? -- Cheers, David